Corner spot where three faces meet
I would make this with linked duplications that I would join later on, but you could do it with 2 Mirror modifiers:


Create your shape. Put an empty on the axis.
Give your shape a Mirror modifier, choose the empty as Mirror Object.
Rotate the empty 45° on the Z axis.
Create a second empty (perhaps another shape) at the exact same place as the first one.
Give your shape a second Mirror modifier, choose the second empty as Mirror Object.
Rotate the second empty 45° on the Y axis.
You should be right.